In a traditional accessory drive, a loose branch of the drive remains loose both during operation of the internal combustion engine and during starting of the latter. In particular, the expression loose branch indicates the branch arranged between two pulleys, the mean tension of which in use is lower than that of all the other branches. Normally the loose branch of a traditional accessory drive is arranged immediately downstream of the crankshaft pulley with respect to the belt rotation direction. Furthermore, in a traditional accessory drive, the electric machine is an alternator which is driven either by the crankshaft pulley or by the starter motor.
In a start-stop accessory drive, on the other hand, the torque transmitted varies when the internal combustion engine drives the belt and when the belt is driven by a motor generator connected to the drive to start the internal combustion engine.
When the torque acting on the belt changes direction, a tight branch of the belt drive can become loose and vice versa and the belt tension must be checked to prevent slipping during all the operating conditions of the drive and in particular of the transients.
A traditional tensioner must be sized to guarantee correct tensioning of the belt in the most severe conditions, i.e. during starting, but said tension is excessive during the other operating conditions, for example when the vehicle is running. In particular, the belt tension impacts on the sizing of further engine components, such as the shaft bearings of the accessories connected to the drive and on the friction dissipation of the drive. It is therefore preferable for the belt tension to be kept as low as possible compatibly with the operating conditions.